When no error message needs to be passed, use 'return' to exit the
generator as recommended in PEP-479.

When an error message needs to be passed, raise a RuntimeError exception
instead to ease the data flow. No need to catch exception in the
iterator as we will deal globally with exceptions raised at higher
level.

The logic within 'util.defer' is catching all exceptions and emit it
as a result without discriminating between finished tasks and errors.
Therefore we could simply check if the result is an instance of
Exception to figure out if the process should exit.

Using try-catch to transmit error messages from the iterator to the
process callback would be nicer, but some issues remains:
- It doesn't account for non-error messages (e.g. "Stopped")
- It doesn't discriminate between error which should be caught and error
  which should raise.

Therefore it is simpler to keep using StopIteration to exit the iterator
while transmitting messages to display, and change the syntax to conform
with Python 2.7 (No 'return' with argument allowed inside generators)
and Python 3.7 (generators shouldn't raise StopIteration).

…g_within_iterator